I have an established tree that does not lose it's leaves in the winter. Sorry I cannot tell you what it is, but it would provide shade for the maples in Summer.

I would like to plant several Japanese Maples (Crimson Queen, Orangeola, Shin Deshojo) beneath that large tree and wonder if the 'volcanic rock' will pose a problem if I don't remove all of it? Would it be wise to dig a deeper hole and put some of that 'volcanic rocks' in the bottom of the hole, to provide more drainage? I would appreciate any suggestions as I'm new to planting those beautiful trees here in Texas.

I would appreciate any suggestions.

I have been using a smaller size of the black lava rock in my maple containers for a while now. I use about 20% lava rock 1/4" size to mix with my container soil to provide aeration. That said, you can just till the lava rocks into the soil, it will only help.
I am assuming that you have clay soil, since that is what I have in Dallas, if this is the case do not dig a hole and then fill it with lighter soil mix. It will create a water basin that will drown the trees. If you have clay soil, it will be better to plant on a mound, or use as much of the native soil as possible.
But if you lucky to have loamy soil, then just till the lava rocks into the soil and plant.

xman

Thanks for your insight on the clay soil. It didn't occur to me that it would create a 'water basin', and I know these trees need good drainage. I have read that the JMs I'm choosing have a shallow but wide root system and I plan to cover the entire area (15' x 15') with light colored river rock (approx 1-1/2" to 3").

I know not to place the rock against the trunks, but i'm wondering how close to place the rocks and how high a mound you would suggest. The large shading tree and the JMs will be the only vegetation in that area.

Another item i'm concerned about. I have a sprinkler system already in place (you already know about the foundation consequences). Is it wise to have water spraying on them, or should I consider drip lines to the trees? My Winter watering schedule is around 8AM and Summer is at 5AM.

Again, I appreciate your feedback.

I would probably use mulch around the tree (mulch should be away least a few inches from the trunk) since mulch would be better in maintaining the temperature and moisture compared to the stones. You could do a 2 feet radius of mulch around the tree, and them use the rock after that.
I have not planted any of my trees on the ground, so I do not have any first hand experience, but the attached link may provide you with some more information.
A sprinkler may not be able to provide the water a tree needs, it may be easier to convert some of the sprinkler heads to drip systems. This is how I water all my containers, I replace sprinkler heads with drip manifolds and then run these to the containers.

Another planting article

http://www.pacificcoastmaples.com/japanese-maples/planting-japanese-ma ple-trees.html

You may want to take into account the adult size of these trees when deciding on how many trees you want to plant in that area. Some of these grow pretty wide.

xman

The websites you posted give me more insight on planting. May I ask how large your pots are for your JMs? I'm considering potting them and placing the pots half buried in the landscape, then bringing the rock right up to the pots. I'll be putting in a drip line like you suggested to each pot, from the sprinkler heads. I'm considering some type of stone pot, granite or marble, versus the clay pots.

Thanks for your reply.

Growing them in containers has it own pros and cons.
Pros: You can control the soil mix, and the drainage. You can move the tree around to the best suitable position in terms of sun and wind.
Cons: You need to really keep the watering schedule. The trees need regular repotting and root pruning.

You may not be able to bury the pots if you have clay soil. Look into a raised bed as an option.
Most of my trees are in 7 gallons size, I have a few in the
20 - 25 gallon size, and some smaller. I do not want to go more than 25 gallon size, as it get very heavy.
I grow all my trees in grow bags (smart pots). I place these bags in clay containers, or whatever containers I can get for cheap. Large containers, even clay one are expensive, I usually get ones that have hairline cracks in them for a few bucks and fix the cracks. They usually hold up pretty well since the root bag takes all the pressure of the soil etc.

With the 15 x 15 place, you can probably some nice raised beds. You can try one large bed, or many smaller beds different shapes, using pavestone, etc. or plant on a mound and companion plant around the mound.

xman